Neurotoxins that target ion channels are among the chemicals that NIH has identified as at the nexus of neuroscience research and counterterrorism. Archetypal examples in this regard are the alkaloids tetrodotoxin (TTX) and saxitoxin (SIX), both potent paralytic agents that act by binding to the same site on, and blocking the function of voltage-gated sodium channels. We propose to develop a conceptually novel antagonist of these and related neurotoxins. The concept is encapsulated by the term contratoxin, defined as follows: an agent that binds the target of a toxin and shields the target from the toxin. To provide proof-of-concept contratoxin data, we will utilize conotoxins, peptide-based neurotoxins produced by venomous cone snails. Their exquisite potency and selectivity toward individual ion channels or receptors have been shown in many instances. In this exploratory proposal, our experimental strategy consists of three parallel approaches to obtain contratoxins against TTX/STX: (1) rational design, (2) molecular biology/chemical synthesis and (3) screening natural product libraries. In each aim, candidate contratoxins will be tested for their inability to block sodium channels on one hand and their ability to block the action of TTX/STX on the other. In the Specific Aim I, we will rationally design conotoxin GIIIA-based analogs, followed by a chemical synthesis and functional evaluation. These so-called "leaky" analogs will target the TTX/STX binding site on sodium channels, but void paralytic activity. In addition, we propose to screen for a contratoxin activity among selected conotoxins discovered by molecular cloning or/and from fractionated venom components of invertebrate-hunting cone snails (Specific Aims II and III, respectively). Our long- term goal is to develop contratoxins as novel tools that will be utilized across multiple fields, including neuroscience, neuropharmacology and medical countermeasures against neurotoxic agents. This research is also expected to illuminate the way for investigations of other contratoxins, such as those against TTX/STX on neuronal sodium channels as well as against other neurotoxins with other ion channel targets. [unreadable] [unreadable] [unreadable]